1. Week 4 - Friday
CS222

2. Last time What did we talk about last time? Functions
Systems programming

3. Questions?

4. Project 2

5. Quotes Unix never says "please." It also never says: Rob Pike
"Thank you"
"You're welcome"
"I'm sorry"
"Are you sure you want to do that?"

6. More Systems Programming Stuff

7. Users and groups
On Linux, every user has a unique login name (user name) and a corresponding numerical ID (UID)
A file (/etc/passwd) contains the following for all users:
Group ID: first group of which the user is a member
Home directory: starting directory when the user logs in
Login shell
Groups of users exist for administrative purposes and are defined in the /etc/group file

8. Superusers The superuser account has complete control over everything
This account is allowed to do anything, access any file
On Unix systems, the superuser account is usually called root
If you are a system administrator, it is recommended that you do not stay logged in as root
If you ever get a virus, it can destroy everything
Instead, administrators should log in to a normal account and periodically issue commands with elevated permission (often by using sudo)

10. Single file system
In Windows, each drive has its own directory hierarchy
C: etc.
In Linux, the top of the file system is the root directory /
Everything (including drives, usually mounted in /mnt) is under the top directory
/bin is for programs
/etc is for configuration
/usr is for user programs
/boot is for boot information
/dev is for devices
/home is for user home directories

11. Files
There are regular files in Linux which you can further break down into data files and executables (although Linux treats them the same)
A directory is a special kind of file that lists other files
Links in Linux are kind of like shortcuts in Windows
There are hard links and soft links (or symbolic links)
File names can be up to 255 characters long
Can contain any ASCII characters except / and the null character \0
For readability and compatibility, they should only use letters, digits, the hyphen, underscore, and dot
Pathnames describe a location of a file
They can start with / making them absolute paths
Or they are relative paths with respect to the current working directory

12. File permissions
Every file has a UID and GID specifying the user who owns the file and the group the file belongs to
For each file, permissions are set that specify:
Whether the owner can read, write, or execute it
Whether other members of the group can read, write, or execute it
Whether anyone else on the system can read, write, or execute it
The chmod command changes these settings (u is for owner, g is for group, and o is everyone else)

13. File I/O All I/O operations in Linux are treated like file I/O
Printing to the screen is writing to a special file called stdout
Reading from the keyboard is reading from a special file called stdin
When we get the basic functions needed to open, read, and write files, we'll be able to do almost any kind of I/O

14. Processes A process is a program that is currently executing
In memory, processes have the following segments:
Text The executable code
Data Static variables
Heap Dynamically allocated variables
Stack Area that grows and shrinks with function calls
A segmentation fault is when your code tries to access a segment it's not supposed to
A process generally executes with the same privileges as the user who started it

15. Scope

16. Scope
The scope of a name is the part of the program where that name is visible
In Java, scope could get complex
Local variables, class variables, member variables,
Inner classes
Static vs. non-static
Visibility issues with public, private, protected, and default
C is simpler
Local variables
Global variables

17. Local scope
Local variables and function arguments are in scope for the life of the function call
They are also called automatic variables
They come into existence on the stack on a function call
Then disappear when the function returns
Local variables can hide global variables

18. Global scope
Variables declared outside of any function are global variables
They exist for the life of the program
You can keep data inside global variables between function calls
They are similar to static members in Java
int value;
void change() {
value = 7; }
int main()
value = 5;
change();
printf("Value: %d\n", value);
return 0;

19. Use of global variables
Global variables should rarely be used
Multiple functions can write to them, allowing inconsistent values
Local variables can hide global variables, leading programmers to think they are changing a variable other than the one they are
Code is much easier to understand if it is based on input values going into a function and output values getting returned

20. Hiding
If there are multiple variables with the same name, the one declared in the current block will be used
If there is no such variable declared in the current block, the compiler will look outward one block at a time until it finds it
Multiple variables can have the same name if they are declared at different scope levels
When an inner variable is used instead of an outer variable with the same name, it hides or shadows the outer variable
Global variables are used only when nothing else matches
Minimize variable hiding to avoid confusion

21. extern declarations
What if you want to use a global variable declared in another file?
No problem, just put extern before the variable declaration in your file
There should only be one true declaration, but there can be many extern declarations referencing it
Function prototypes are implicitly extern
file2.c
int count;
extern int count;
file1.c
file3.c
program.c

22. static declarations
The static keyword causes confusion in Java because it means a couple of different (but related) things
In C, the static keyword is used differently, but also for two confusing things
Global static declarations
Local static declarations

23. Global static variables
When the static modifier is applied to a global variable, that variable cannot be accessed in other files
A global static variable cannot be referred to as an extern in some other file
If multiple files use the same global variable, each variable must be static or an extern referring to a single real variable
Otherwise, the linker will complain that it's got variables with the same name

24. Local static variables
You can also declare a static variable local to a function
These variables exist for the lifetime of the program, but are only visible inside the method
Some people use these for bizarre tricks in recursive functions
Try not to use them!
Like all global variables, they make code harder to reason about
They are not thread safe

25. Local static example #include <stdio.h> void unexpected() {
static int count = 0;
count++;
printf("Count: %d", count); }
int main()
unexpected(); //Count: 1
unexpected(); //Count: 2
unexpected(); //Count: 3
return 0;

26. The register modifier register int value;
You can also use the register keyword when declaring a local variable
It is a sign to the compiler that you think this variable will be used a lot and should be kept in a register
It's only a suggestion
You can not use the reference operator (which we haven't talked about yet) to retrieve the address of a register variable
Modern compilers are better at register allocation than humans usually are
register int value;

27. Compiling multiple files
All real programs are written in multiple files
To compile such files, do the following
Create a header file (with a .h extension) for every file that contains prototypes for functions you're going to use in other files
#include those header files in every file that uses them
When you run gcc, put all the .c files needed on the line at the same time

28. Multiple compilation example
Your main() function and core program is in a file called program.c
You have files called networking.c and graphics.c that have networking and graphics functions that your program uses
You should have headers called networking.h and graphics.h (names don't have to match, but it is better if they do)
At the top of program.c should be:
To run gcc you type:
#include "networking.h"
#include "graphics.h"
gcc program.c networking.c graphics.c -o program

29. Or you can use compile but not link
You can compile a file into object code without linking it into an executable
Produces a .o file
That way, you can compile all the pieces separately and then link them later
This can be more efficient if you are updating some of the code but not all of it
To compile but not link, use gcc -c
We could compile the previous example as follows
gcc –c program.c
gcc –c networking.c
gcc –c graphics.c
gcc program.o networking.o graphics.o -o program

30. Makefiles to the rescue
Now that we're talking about compiling multiple files, a makefile really makes (ha, ha) sense
all: program
program: program.o networking.o graphics.o
gcc program.o networking.o graphics.o -o program
program.o: program.c networking.h graphics.h
gcc –c program.c
networking.o: networking.c
gcc –c networking.c
graphics.o: graphics.c
gcc –c graphics.c
clean:
rm -f *.o program

31. Lab 4

32. Upcoming

33. Next time…
Processes
Arrays

34. Reminders
Read K&R chapter 5
Keep working on Project 2